1. Field of the Invention
The present invention is related to a gradation display method in an image output-apparatus. More specifically, the present invention is directed to a gradation display method by that an image to be displayed is divided into pixels having fine areas, and this pixel is furthermore divided into very fine pixels having very fine areas, and then, gradation is represented based upon a ratio of a colored area of a colored very fine pixel with respect to areas of all very fine pixels within the above-described pixel.
2. Description of the Related Art
Conventionally, when images having gradation are displayed in image output apparatus such as printing machines, printers, and digital type copying machines, such methods for displaying the gradation in a quasi-gradation display manner have been employed.
In the above-described quasi-gradation display method, gradation is displayed, or represented in such a manner that an image is divided into fine unit pixels, variable density is resembled in a continuous tone based upon large/small areas occupied by colored very fine elements among very fine pixels (for instance, either points or lines) within this unit pixel.
Then, such methods have been employed many times by which the gradation is displayed by large/small mesh points which are formed by the colored very fine elements within the unit pixel.
As the above-described method using the mesh point, a density pattern method (namely, area gradation method) is known. This density pattern method corresponds to such a method that while one pixel on the display side (on the side of image output apparatus) corresponding to one pixel of an original image is divided into a plurality of very fine pixels, a predetermined number of very fine pixels corresponding to gradation of the pixel are selected from a plurality of very fine pixels, and these selected very fine pixels are colored by using a predetermined color (for example, black). In this method, mesh points are formed from a predetermined number of colored very fine pixels corresponding to the above-described gradation.
In the above-described density pattern method, gradation having plural stages in response to the total number of very fine pixels which constitute one pixel on the above-described display side can be displayed.
FIG. 27 is an explanatory diagram for explaining prior art as to colored very fine pixels in the case that while a total number of very fine pixel “Si” which constitute the above-explained one pixel “S” is defined as 4×8=32, a binary display is performed based upon the respective very fine pixels Si (i=1 to 32). FIG. 27A is an explanatory diagram for indicating fourth gradation of colored very fine pixels S1 to S4. FIG. 27B is an explanatory diagram for showing eighth gradation of colored very fine pixels S1 to S8.
FIG. 27A to FIG. 30B are explanatory diagrams for explaining 12th gradation of colored very fine pixels S1 to S12; 16th gradation of colored very fine pixels S1 to S16; 20th gradation of colored very fine pixels S1 to S20; 28th gradation of colored very fine pixels S1 to S28; and 32-nd gradation of colored very fine pixels S1 to S32, respectively.
As a pixel “S” shown in FIG. 27 to FIG. 30B, a screen angle (namely, screen angle of colored very fine pixels) “θ” with respect to a horizontal line direction is equal to 45 degrees, and this screen angle “θ” corresponds to a straight line which connects centers of the above-described pixels “S”.
Then, a growth direction (increasing direction of colored very fine pixels, namely, a growth direction of growth core) as to the first gradation to the 32-nd gradation of colored very fine pixels is set to the same direction as the screen angle.
The pixels “S” indicated in FIG. 27 to FIG. 30B are line growth type pixels, and thus, the colored very fine pixels are grown along the screen angle.
When an image is formed, in the case that the above-described gradation is displayed with respect to narrow lines, characters, and the like, these narrow lines/characters are easily blurred, and/or broken. In particular; this problem may readily become conspicuous at a screen angle, and at a line at an angle approximated to the screen angle. As a method of avoiding this problem, the below-mentioned methods (J01) to (J03) are known in this field.
The method capable of increasing the screen line number (J01):
In this method (J01), the above-described problem can be improved, but IOT defects (noise emphasis, uniformity within plane of density) are deteriorated as a secondary failure.
The technique described in JP-B-7-105888 (J02):
This conventional technique described in this publication may avoid a blurring phenomenon of lines and a broken phenomenon of lines by such a manner that a narrow line is recognized, this recognized narrow line is subdivided into very small blocks, and then, mesh point shapes (screen patterns) are produced which are different from each other every very small block.
However, in this conventional technique (J02), the process operation becomes complex, and thus, the manufacturing cost of the process circuit is increased. Also, the data processing amount is considerably large, and thus, the processing speed is lowered.
The conventional technique described in JP-A-8-156329:
In this technique described in this patent application, an identification is made of as to whether an image signal corresponds to a line image or a natural image. Based upon an identification result, two sets, or more sets of density signal converting means are switched.
In accordance with this technique (J03), the identifying means for identifying the image signal corresponds to the line image, or the natural image, and also, two sets, or more sets of density signal converting means are required, resulting in higher cost.